There are many types of fuel cell depending on the type of electrolyte used, such as polymer, phosphoric acid, solid oxide, molten carbonate, and alkaline. Among these, polymer electrolyte fuel cells have lower operating temperature and shorter startup time than other types of fuel cell. They are also easier to produce high output, expected to be reduced in size and weight, and resistant to vibration. For these reasons, the polymer electrolyte fuel cell is suitable as a power source for mobile objects.
Fuel cells generally employ a perfluorosulfonic acid membrane having a high proton conductivity. Examples of ion-exchange membrane currently used in polymer electrolyte fuel cells include perfluorocarbon sulfonic acid membranes such as Nafion (registered trademark) by DuPont, Flemion (registered trademark) by Asahi Glass Co., Ltd, Aciplex (registered trademark) by Asahi Kasei Corporation.
When such ion-exchange membrane is applied in a polymer electrolyte fuel cell, a membrane-electrode assembly is used which has a structure such that electrode catalyst layers having a fuel oxidizing capacity or an oxidant reducing capacity are disposed on both sides of the ion-exchange membrane, on the outside of which gas diffusion layers are further disposed.
Specifically, the structure includes an ion-exchange membrane consisting of a polymer electrolyte membrane that selectively transports hydrogen ion, on each side of which the electrode catalyst layer is formed. The electrode catalyst layer comprises, as a main component, a carbon powder supporting a platinum group metal catalyst. On the outer surface of the electrode catalyst layer, the gas diffusion layer, which has both a fuel gas permeability and electron conductivity, is formed. Generally, the gas diffusion layer consists of a substrate of carbon paper or carbon cloth on which a film of a paste containing a powder of fluorine resin, silicon, carbon or the like is formed. The aforementioned electrode catalyst layer and the gas diffusion layer are collectively referred to as an electrode.
In order to prevent the leakage of the supplied fuel gas and the mixing of the two kinds of fuel gas, a gas sealing member or a gasket is disposed around the electrode in such a manner as to sandwich the ion-exchange membrane. The gas sealing member, gasket, electrode, and ion-exchange membrane are assembled in an integrated manner beforehand, into what is called a membrane-electrode assembly (MEA).
On the outside of the MEA, an electrically conductive and airtight separator is disposed for mechanically fixing the assembly and electrically connecting adjacent MEAs to each other in series. A portion of the separator that is in contact with the MEA is formed with a gas channel for supplying a reaction gas to the electrode surface and to carry produced gas or excess gas away. While the gas channel can be provided separately from the separator, generally it is formed by providing a groove in the surface of the separator. Such structure consisting of the MEA fixed by means of a pair of the separators is used as a single cell, which is the basic unit.
By connecting a plurality of such single cells in series and arranging a manifold, which is a piping jig for the supply of fuel gas, a fuel cell is constructed.
Thus, manufacture of a polymer electrolyte fuel cell, particularly the electrode consisting of an electrode catalyst layer and a gas diffusion layer, requires complex process steps and technology. Furthermore, since the carbon powder supporting a platinum group metal catalyst does not necessarily have a large specific surface area, the amount of the supported platinum group metal catalyst is large, inevitably resulting in high cost.
In addition to causing the electrode catalyst layer (either a cathode catalyst layer or an anode catalyst layer) to retain a sufficient amount of catalyst particle for obtaining the catalytic function, electron conductivity is required between the electrode catalyst layer and the separator, which is a current collector, and proton conductivity is required between the electrode catalyst layer and the electrolyte membrane. Therefore, conventionally a catalyst layer on the order of several 10 g/m has been formed of a mixture of catalyst-supported conductive particle having a particle diameter on the order of 50 nm and proton conductor.
In the electrode catalyst layer of such structure, the electrons formed in the catalyst near the electrolyte membrane, for example, do not reach the current collector unless they move between a plurality of conductive particles. However, the area of contact between the conductive particles is small; in some cases, the electric resistance between the conductive particles is high because of the presence of the proton conductivity material between the particles. Thus, in the conventional catalyst layer, the electron conductivity between the current collector and the electrode catalyst layer is low, resulting in a decrease in the generating efficiency of the fuel cell.
The electron conductivity between the current collector and the electrode catalyst layer can be increased by increasing the density of the catalyst layer; however, increasing the density of the catalyst layer leads to a decrease in the dispersibility of fuel or oxidant into the catalyst layer, thereby making it impossible to fully exploit the catalytic function of the catalyst particle.
Concerning a technology related to the catalyst layer, it is reported in Electrochem. Acta., vol. 38, No. 6, p. 793 (1993) that carbon fiber was used as a catalyst carrier, where a catalyst particle was supported on the surface of the carbon fiber. However, if a carbon fiber that carries a catalyst particle is fabricated and the fiber is formed on the surface of the current collector to form an electrode which is used in a fuel cell, although the probability of the electrons generated near the electrolyte membrane moving between the particles (fibers) before they arrive at the current collector may become smaller, several times of transfers between the particles would be normally required, which will make it difficult to increase the electron conductivity sufficiently.
Thus, it has been difficult with the conventional electrodes for fuel cells to increase the electric conductivity of the catalyst layer to sufficient levels, and so it has been impossible to achieve a sufficiently high fuel cell generation efficiency. JP Patent Publication (Kokai) No. 2002-298861 A discloses an invention of an electrode for fuel cells, its object being the provision of a fuel cell having a high generation efficiency, a fuel cell electrode for realizing such fuel cell, and a method for manufacturing a fuel cell electrode achieving such fuel cell. This electrode comprises a current collector made of a conductive porous material, and a catalyst layer comprised of carbon nanofiber of which 50% or more of the tip portions has an angle of elevation of 45° or more with respect to the plane of the current collector, an electrode catalyst particle supported on the carbon nanofiber surface, and a proton conductor formed on the surface of the carbon nanofiber in contact with the electrode catalyst particle.